Having watched Brown, there is no doubt that he has very slow feet which he compensated in college by being strong and massive. That won't work as well in the NFL. That said, I would like to see if coaching could improve and compensate for his obvious deficiency.
I have to agree. I've never understood the hype about Brown. I've watched nearly a dozen of his college games (actual games, not highlights) and not a single time have I seen any glimpse of a NFL LT.
He's obviously massive with long arms. He doesn't get mauled by bull rushers because he's so big he can just absorb them into his body without losing balance. OU did a good job of taking away the inside pass rushing gap, so the only way to get to the QB was all the way around the corner. Brown is so long that this was nearly impossible unless the rusher was an elite athlete. He didn't face that many. The problem at the next level is that all the weak side pass rushers will be elite athletes. All capable of neutralizing his length and beating him on the edge.
Brown gets beat by speed a ton. Mayfield made these guys look way better than they are. He got rid of the ball very quickly the majority of the time, and he is a very instinctive creator/scrambler when he can't get the ball off on time. The lack of sacks given up by OU should be credited to Mayfield, not the OL.
Brown is very susceptible to speed around the corner. He is just too slow to get out there, especially if the defender bends underneath him. And he also gets killed in this same scenario on an inside counter move. If he does get to the corner, the speed guy can easily use his momentum against him and beat him inside with a club or rip.
He's also off-balance in the run game because he's so dang tall. He has a hard time creating leverage which leads him to drop his head and miss pretty often.
I don't think he's a good enough run blocker to move inside to OG. When he gets his hands on people he murders them. But he's going to struggle getting there. And he's a disaster waiting to happen at LT. His only possible future is to get coached up at RT. He will always be a liability but his pure size and length give him an advantageous starting position when pass blocking. He's a terrible athlete. That won't improve. But if his feet and hands could be more productive he could be serviceable.
I see a 3rd/4th round ceiling of a player (average NFL starter at RT) with a high chance to be a complete bust.
